Enhancement of photoisomerization of Azobenzene dye Chicago Sky Blue 6B (CSB) was attempted using long-range surface plasmon resonance (LRSPR). The CSB film was prepared on a grating substrate with metal film for LRSPR, and its transmission light property was investigated before and after white light irradiation for photoisomerization. The transmission light was markedly changed by irradiation of p-polarized white light, and the change was attributed to photoisomerization of CSB induced by LRSPR which accompanies an intense electric field. Furthermore, the effect of LRSPR was significant compared to standard SPR.